This invention relates to an extrusion molding method for producing a continuous product, in particular a tube with a thin thickness or a thin tube made of a synthetic resin material and also relates to an extrusion molding apparatus for carrying out the method.
Heretofore, an extrusion molding method for a thin tube made of a synthetic resin material has formed a continuous product for a thin tube by plasticizing and kneading a raw material composed of a synthetic resin material by means of an extrusion molding machine, extruding the plasticized and kneaded raw material from a molding die to form a tubular body having a circular cross section, further passing the tubular body on a sizing die to give a final inner diameter to the tubular body, and cooling and hardening the tubular body. The formed material which had passed through the molding die and sizing die fell down naturally or was forcedly drawn (pulled down) and then stacked as it was or taken up onto a reel.
Recently, a high heat-resistance and high strength synthetic resin material such as polyether imide (hereinafter merely referred to xe2x80x9cPEIxe2x80x9d), polyeter ether ketone (hereinafter merely referred to xe2x80x9cPEEKxe2x80x9d), or the like have been used as a material for a voice coil bobbin.
Heretofore, a material for use in a sizing die utilized in an extrusion molding apparatus for a thin tube made of a most common synthetic resin material is a metal material having a high heat transfer rate and a rich lubrication, for example, brass or the like. Brass includes many additives such as a filling material and causes a roughness in grain size level on a contacting surface of a sizing die. This roughness of surface gives a very excellent character to a sizing die for extrusion molding a thin tube made of a common synthetic resin material. Also, brass can give the same effect to the sizing die, although the surface finishing accuracy is lowered. However, loading is caused on a contacting surface of the sizing die in use and this lowers durability in wear and stability in size.
On the other hand, a continuous product for a tube made of PEI, PEEK, or the like has a mirror-like finished surface and high mechanical strength. In the case where such continuous product comes into contact with a sizing die having a high heat transfer rate, the continuous product will be instantaneously cooled (in particular, this tendency will often occur in a thin tube.) and wind around the sizing die with a strong force due to its high mechanical strength. This causes a very high frictional force to occur between them. Consequently, the continuous product will not fall freely, or it will be difficult to draw the continuous product. In particular, such tendency will be remarkable in the case where a metallic sizing die has a mirror-like surface.
In order to overcome this problem, a Teflon (trade name; PTFE in a formal abbreviated word) coating may be applied to a contacting surface of a sizing die. However, a synthetic resin material having a high forming temperature such as PEI involves a new problem of heat-resistance in the coating.
Thus, it was almost impossible to form a continuous product for a thin tube made of PEI, PEEK, or the like by means of an extrusion molding method.
Accordingly, an object of the present invention is to provide an extrusion molding method and apparatus for a thin tube made of a synthetic resin material having high heat-resistance and high strength (for example, PEI, PEEK, or the like).
An extrusion molding method in accordance with the present invention forms a continuous product for a thin tube by plasticizing and kneading a raw material composed of a synthetic resin material by means of an extrusion molding machine, extruding the plasticized and kneaded raw material from a molding die to form a tubular body having a circular cross section, further passing the tubular body on a sizing die to give a final inner diameter to the tubular body, cooling and hardening the tubular body. The extrusion molding method is characterized by the steps of coupling the molding die and the sizing die to each other by a connecting pipe; making the sizing die from a carbon material; supplying a first air to an interior of the sizing die through the connecting pipe; and supplying a second air to an interior of the molding die so that an air lubrication layer is formed between an inner surface of the continuous product and an outer surface of the sizing die.
Preferably, the synthetic resin material has high heat-resistance and high strength (for example, polyether imide (PEI), polyeter ether ketone (PEEK), or the like). A frictional resistance caused between the inner surface of the continuous product and the outer surface of the sizing die may be adjusted by controlling flow rate and pressure of the second air.
An extrusion molding apparatus in accordance with the present invention carries out the extrusion molding method wherein a continuous product for a thin tube is formed by plasticizing and kneading a raw material composed of a synthetic resin material by means of an extrusion molding machine, extruding the plasticized and kneaded raw material from a molding die to form a tubular body having a circular cross section, further passing the tubular body on a sizing die to give a final inner diameter to the tubular body, cooling and hardening the tubular body. The apparatus comprises: a connecting pipe for coupling the molding die and the sizing die to each other, the sizing die being made of a carbon material; a first air supplying pipe which passes longitudinally through the connecting pipe and has an upper end coupled to a first air supplying source and a lower end which extends into an exterior of the sizing die and communicates with an interior of the sizing die; and a second air flow passage defined in the interiors of the extrusion molding machine and the molding die so that the second air is supplied to the interior of the continuous product. The first air is exhausted as a third air through the connecting pipe out of the extrusion molding machine after the first air cools the sizing die. The second air passes through a space defined between the inner surface of the continuous product and the outer surface of the sizing die and leaves the continuous product.
The sizing die includes an inner diameter control portion which controls an inner diameter of the continuous product along its longitudinal direction, and a heat quantity adjustment portion which absorbs a residual heat in the continuous production. A ratio (L1/L2) of a length (L1) of the inner diameter control portion of the sizing die to a length (L2) of the heat quantity adjustment portion is preferably set to be within a range of 0.1 to 5.0. A plurality of metallic material rods for reinforcement may be embedded in the sizing die in a manner that the rods extend in a longitudinal direction of the sizing die and are spaced apart from each other in a circumferential direction of the sizing die or a metallic net for reinforcement may be embedded in the sizing die in an annular arrangement. The sizing die may be provided on the heat quantity adjustment portion with a convergently tapered part to adjust a quantity of heat absorption. Preferably, a volume of the tapered part is set to be within a range of 10 to 300% of a volume of an untapered part.
Since the carbon material which constitutes the sizing die is porous, a frictional force on the outer surface of the sizing die is low. Consequently, there is little change of the dimension of the sizing die due to wear and tear. Further, an air in micro cavities on the outer surface of the sizing die will inflate simultaneously when the air comes into contact with the continuous product, and thus results in a separation effect between the continuous product and the outer surface of the sizing die. Since the contacting area of the carbon material is smaller than that of the metallic material although the heat transfer rate of the carbon material is the same as that of the metallic material, a cooling speed for the continuous product is low, whereby a force which clamps the sizing die by the continuous product becomes small. The above effect will be continued, since the interior of the sizing die is cooled by the air.